Fabrication of advanced three-dimensional semiconductor structures with complex surface topology and high packing density presents many technical challenges. In certain embodiments, it is desirable to planarize the surface of a workpiece, to make the top surface as flat as possible. In other embodiments, it is desirable to raise or recess certain portions of the workpiece with respect to the rest of the workpiece.
These planarization processes are most typically conducted using one of two techniques.
The first technique, known as chemical mechanical planarization (CMP), applies a chemical slurry to the workpiece and uses a polishing pad to remove material to flatten the top surface of the workpiece. However, CMP has some known drawbacks. For example, CMP is optimized for a particular width of metal on a workpiece. If there are different metal widths, the planarization across these different widths may not be uniform. Additionally, when planarizing workpieces having metal and dielectric materials on the top surface, a phenomenon known as dishing may occur. When planarizing, the metal may be more readily removed than the dielectric material. Thus, as the process continues, more metal is removed than dielectric material. Further, the metal at the midpoint of the trace is processed more than the metal at the interface with the dielectric material. This causes the top surfaces of the metal widths to form concave shapes, or dishes. Thus, CMP is sensitive to feature width and the types of material that are disposed on the workpiece.
The second technique, know as reactive ion etching (RIE), uses ions from a plasma to etch the workpiece. The workpiece is biased to attract ions from the plasma. These ions chemically react with the workpiece and also cause some sputtering due to their velocity at impact. However, RIE has been demonstrated to suffer from some issues of uniformity, such as center to edge uniformity. Further, RIE is an inherently chemical process, and may have issues of repeatability.
Therefore, it would be beneficial if there would a method of removing material from a workpiece, which could be tuned based on the type and amount of material to be removed. It would be advantageous if the method could be used not only to planarize a workpiece, but also to create recesses and structures as desired. Further, it would also be beneficial if the method was highly repeatable.